It varies massively from part to part, I work mainly within the oil&gas and nuclear, the two are treated very differently with regards to procedures and specifications, generally within castings your looking for shrinkage, porosity, laps, cracks&tears, inclusions, aswell as testing welds etc.
Minimum detectable size can be down to a single gas pore, it comes down to grain structure, material etc, and type of defect. Shrinkage doesn’t give a clean signal, tends to reflect it off all over the place, your looking more for a loss in backwall generally 50%, whereas a crack will go off the screen. so there isn’t really a minimum detectable defect, it varies too much. With regards to specs, it varies massively again, depending on job the things can be tested 2 or 3 times on ut and 6-8 times for mpi, 100% initial, test of excavations, test of repairs, 100% test after post weld heat treatment , 100% test after machining, and then a witnessed test by customer, it all depends in the job in hand, a good example is mpi, one specification states 1.6mm linears to be considered relevant and are to be repaired, to the next job being anything under 13mm linear is acceptable.